Conventionally known as an optical filter having a transmission characteristic with respect to light in a UV-region is, for example, one made by SCHOTT Corporation (catalog number: 3555eIX/84) having transmission characteristics such as those shown in FIG. 1. While FIG. 1 shows transmission characteristics of four kinds of optical filters, each tends to exhibit transmission peaks in both UV- and IR-regions.
On the other hand, silver films have been known to exhibit excellent spectral reflection characteristics with respect to light excluding the UV-region. Known as an optical filter utilizing such a silver film is, for example, one using a silver film as a reflecting mirror disclosed in Japanese Patent Application Laid-Open No. SHO 60-252303. This optical filter comprises such a structure that the silver film absorbs light in the UV-region but reflects light in the other wavelength regions, whereas light in the IR-region from the reflected light is blocked by use of a prism, so as to detect visible light.